This invention relates generally to pulp manufacturing processes and equipment, and more particularly to an apparatus and method for fluffing high consistency pulp and for promoting intimate contact between high consistency pulp and a gaseous bleaching reagent.
As is known, wood pulp is obtained from the digestion of wood chips, from repulping recycled paper, or from other sources and is commonly processed in pulp and paper mills in slurry form in water. Recently there have been many efforts to use ozone as a bleaching agent for high consistency wood pulp, and other lignocellulosic materials, to avoid the use of chlorine in such bleaching processes. Although ozone may initially appear to be an ideal material for bleaching lignocellulosic materials, the exceptional oxidative properties of ozone and its relatively high cost have limited the development of satisfactory devices and processes for ozone bleaching of lignocellulosic materials.
As used herein, the term consistency is used to express the measured ratio of dry pulp fibers to water, or more specifically, the weight of dry pulp fibers in a given weight of pulp slurry or "pulp stock", as a percentage. Various definitions are used, such as air-dry consistency (a.d. %), or oven-dry consistency (o.d. %) or moisture-free consistency (m.f. %). The laboratory techniques for measuring these values can be found in references well known in the art, such as, for example the TAPPI Standards Manual. Terms widely used to describe ranges of stock consistency useful in pulp and paper plants follow:
Low Consistency--Below about 4-6% o.d. PA1 Medium Consistency--About 9-18% o.d. PA1 High Consistency--Above about 18-20% o.d., but more commonly above about 25% o.d.
The primary characteristic of pulp slurries which changes with the consistency of the slurry is the fluidity. Wood pulp in the high consistency ranges does not have a slurry like character, but is better described as a damp, fibrous solid mass. High consistency pulp has an additional characteristic which is that it can be fluffed, in the same way that dry fibrous solids such as cotton or feathers can be fluffed, to give the pulp a light and porous mass, the inner fibers of which are accessible to a chemical reagent in gaseous form. In general, high consistency pulp can not be pumped in pipelines because the pipe wall friction is very high, resulting in uneconomic pumping power requirements. In the specialized case of feeding a gaseous bleaching reactor, such as ozone, it has proved practical to feed high consistency pulp wood with a screw through a short length of pipe to form an impervious plug for sealing against loss of gas.
When fluffed with a fluffing machine, such as a high consistency refiner or a pin mill for example, the high consistency fluffed pulp form a fragile fibrous mass of highly variable bulk density, the latter depending on how it is handled at the discharge of the fluffer. If for example, it is discharged into a shallow bin onto a floor, it will form a pile of fluffed pulp, and if the accumulated pile of fluffed pulp is allowed to build up to a height of about 10 feet, the weight of the pulp is sufficient to compress the fluffed pulp at the bottom of the pile to thereby reduce the gas volume within the fluffed pulp. This characteristic of compressibility of fluffed pulp makes it difficult to move or to transport fluffed pulp in conventional solids bulk handling equipment without increasing the bulk density and reducing the porosity (void volume), which has major implications in equipment for gaseous bleaching.
It is known that to realize fully the advantages of the gas phase reaction in a multi-stage bleaching of cellulosic fibrous pulp, the comminution of the pulp to produce the fluffed pulp must be of a specific nature so as to produce fragments which independent of their size are of low density, and of porous structure throughout and substantially free from any highly compressed portions, i.e. compacted fibre bundles. Only when this form of comminuted pulp is achieved can the gaseous reactants reach all parts of the comminuted pulp fragments, and thus ensure that the reaction of the gaseous reagent with the fluffed pulp proceeds rapidly and uniformly. The concern for uniformity of contact between the fluffed pulp and the bleaching reagent gas, in the case of ozone bleaching, is fostered by the rapid reduction in the concentration of ozone gas in contact with the fluffed pulp. This reduction is attributable to the extremely fast reaction rate of ozone with wood pulp. Since the reaction rate is concentration dependent, this characteristic increases the non-uniform bleaching results attendant upon the variable permeability of the pulp.
As described hereinabove, the fluffed pulp mass is easily compressed by the action of bulk solids handling equipment to form wads and clumps having much higher density and much lower gas permeability. Bleaching gas flows much more slowly through such wads and clumps and much more rapidly through the wad-to-wad contact areas. The result is overbleached contact areas and underbleached wad cores. Thus, it has been found that bleaching systems which employ conventional bulk materials handling equipment to move the fluffed pulp through a bleaching retention chamber while bleaching it with ozone gas cannot successfully produce uniformly bleached pulp fluff.
Pin shredders and fluffers are used in pulp and paper manufacture and in many other industries for shredding sheet material or fluffing fibrous materials. Typically, in these machines, a sheet of wood pulp at a consistency of about 15-50% is received in a radially inward direction by a pin roll which is equipped with an array of small pins which tear off small particles of pulp and fling them down into a collecting conveyor or chute for further processing. The size of the particle produced by such a pin shredder depends on the size and spacing of the pins and the speed of rotation.
When a very fine particle of pulp is desired, as for example in the flash drying of wood pulp or in gas phase high consistency bleaching, machines have been tried which enclose a pin rotor in a housing, except for a feed chute and a discharge opening. An example of such a machine is a fluffer used in high consistency bleaching experiments, and which is described in U.S. Pat. No. 3,725,193 to De Montigny. This machine includes a chute at the top of a cylindrical housing which encloses a pin rotor. Bulk pulp is fed to the machine through the chute. The bulk pulp is ripped apart on coming in contact with the pins of the pin rotor. The bulk pulp is further reduced in particle size as it is carried repeatedly around the interior of the housing. This machine is also equipped with slots or a screen at a housing bottom which permit sufficiently small particles or individual fibers to be discharged, but retain larger particles for further defibration. However, while this machine, and other similar machines, may have operated with varying degrees of success, these machines suffer from a plurality of shortcomings which have detracted from their usefulness.
For example, a disadvantage of using a screen to retain the coarse particles within the housing arises from the fibrous and floccular nature of moist wood pulp. More particularly, with softwood or coniferous wood pulps, whose fibers may average 2.5-3.5 millimeters in length, there is a strong tendency for the fibers which have been separated to aggregate into clumps commonly called flocs, and which may be much larger than the fibers themselves. For the flocs to pass through the screen, the apertures or slots must be undesirably large, which will result in permitting unfluffed particles of similar size to pass.
Another disadvantage of present pin rotors for use in fine fluffing moist wood pulp is the tendency of fibers to collect on the tips of the pins and adhere to the pins, thereby forming a lump of wood pulp which effectively enlarges the size of the pin at the tip. Such a lumping of wood pulp prevents the small pin tip from tearing away small pieces of pulp. Additionally, such lumping of wood pulp at a pin rotor tip leads to bridging between adjacent pins and may produce a jamming action which can bend the pins or stall the rotor. As a result, these machines have proven to be useful only when charged with a small amount of wood pulp and confined to laboratory use. More particularly, experimentation has shown that charges of pulp in excess of about 30 grams of high consistency wood pulp will cause sufficient bridging to create a frictional drag in the machine housing of sufficient magnitude to bend the individual pins.
In addition to the foregoing, and in present pin rotor machines for operation in the high speed range for processing high consistency wood pulp, typically the present high speed pin rotor machines are equipped both with rotating pins disposed on the rotor and stationary pins disposed on the interior housing wall. Such high speed pin rotor machines have operated with varying degrees of success in the low to medium consistency ranges for processing wood pulp. However, these high speed pin rotor machines are replete with shortcomings which have detracted from their usefulness in processing high consistency wood pulp. For example, theses machines experience severe plugging during operation by operation of the wood pulp fibers wrapping against the stationary pins and being trapped thereon by the centrifugal force of the operating machine.
The foregoing illustrates limitations known to exist in present machines for fluffing and manipulating high consistency wood pulp. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.